Base-catalyzed oxyalkylation has been used to prepare polyoxyalkylene polyols for many years. In such a process, a suitably hydric starter molecule is oxyalkylated with one or more alkylene oxides, such as ethylene oxide (“EO”) or propylene oxide (“PO”), to form a polyoxyalkylene polyether polyol product. Strongly basic catalysts such as sodium hydroxide or potassium hydroxide are typically used in such oxyalkylations.
Thus, most of polyoxyalkylene polyols useful in synthesis of polyurethane polymers, as well as those suitable for other uses, contain substantial amounts of oxypropylene moieties. As those skilled in the art are aware, during base-catalyzed oxypropylation, a competing rearrangement of propylene oxide to allyl alcohol generates monofunctional species which also become oxyalkylated, producing a wide range of polyoxyalkylene monols with molecular weights ranging from that of allyl alcohol itself or its low molecular weight oxyalkylated oligomers to polyether monols of very high molecular weight. In addition to broadening the molecular weight distribution of the product, the continuous generation of monols lowers the product functionality.
The monol content of polyoxyalkylene polyols is generally determined by measuring the unsaturation, for example by ASTM D-2849-69, “Testing of Urethane Foam Polyol Raw Materials”, as each monol molecule contains allylic termination. Levels of unsaturation of about 0.025 meq/g to in excess of 0.10 meq/g for based-catalyzed polyols such as those described above are generally obtained. Numerous attempts have been made to lower unsaturation, and hence monol content, but few have been successful.
In the early 1960's, double metal cyanide (“DMC”) complexes, such as the non-stoichiometric glyme complexes of zinc hexacyanocobaltate, were found which were able to prepare polyoxypropylene polyols with low monol contents, as reflected by unsaturation in the range of 0.012 to 0.020 meq/g. This represented a considerable improvement over the monol content obtainable by base catalysis.
In the 1970's, General Tire & Rubber Company, in U.S. Pat. No. 3,829,505, described the preparation of high molecular weight diols, triols etc., using double metal cyanide catalysts. However, the catalyst activity, coupled with catalyst cost and the difficulty of removing catalyst residues from the polyol product, prevented commercialization of the products.
In the 1980's, interest in such catalysts resurfaced, and improved catalysts with higher activity coupled with improved methods of catalyst removal allowed commercialization for a short time. The polyols also exhibited somewhat lower monol content, as reflected by unsaturation values in the range of 0.012 to 0.018 meq/g. However, the economics of the process were marginal, and in many cases, improvements expected in polymer products due to higher functionality and higher polyol molecular weight did not materialize.
In the 1990's, DMC catalysts were developed with far greater activity than was theretofore possible. Those catalysts, described for example in U.S. Pat. Nos. 5,470,813 and 5,482,908, allowed commercialization of DMC-catalyzed polyether polyols. Unlike the low (0.012-0.018 meq/g) unsaturation polyols prepared by prior DMC catalysts, these ultra-low unsaturation polyols often demonstrated dramatic improvements in polymer properties, although formulations were often different from the formulations useful with conventional polyols. These polyols typically have unsaturation in the range of 0.002 to 0.008 meq/g.
In U.S. Published Patent Application 2005-0159628 A1, Stosser et al. disclose that DMC-catalyzed reactions of C13 alcohols with either butylene oxide (“BO”) or propylene oxide produce monols containing surprisingly high levels of unsaturation. The following table reports the calculated unsaturation based upon the data (unsaturation mole %) taken from Table 1, at page 4 of Stosser et al. and charge factors.
CalculatedExp.AlkyleneUnsaturationUnsaturationNo.CatalystOxide(Mole %)(meq/g)1KOHPO<1<0.0062DMCBO28.80.2273DMCBO210.1494DMCBO28.10.2185DMCBO27.10.2076DMCBO14.10.0917DMCPO4.20.04
These calculated unsaturation values are surprising because, as mentioned hereinabove, one of the key attributes of DMC catalysis is the production of polyether polyols with low levels of unsaturation. Typical unsaturation levels for DMC-based propylene oxide polyols are in the range of 0.003 to 0.012 meq unsaturation/g, and the corresponding butylene oxide-based products are in the range of 0.02 to 0.04 meq unsaturation/g. The disclosure of Stosser et al. is silent as to whether any special conditions were responsible for producing these high levels of unsaturation. Further, Stosser et al. do not teach how to control the production of the by-product unsaturation. As mentioned hereinabove, high levels of unsaturation are undesirable because the allyl or methallyl by-products can alter the characteristics of the resultant polyethers.
Although the present inventors have noticed that high levels of unsaturation are obtained when using certain C13 alcohols containing trace amounts of boron compounds, other alcohols, such those from Shell's NEODOL series do not contain these boron residues and produce polyether polyols having unsaturation values in the range noted above. It appears that the problematic alcohols were treated with either sodium or potassium borohydride to prevent color formation during or after production and these residues interact with the DMC catalyst to cause the formation of the high levels of allylic alcohols. The polyether monols containing high levels of unsaturation are less desirable because a large fraction of the monols is initiated with allyl alcohols instead of with the C13 alcohol. The C13 alcohol-based product is desirable in certain application such as deposit control additives as the larger alkyl group is a major contributor to solubility characteristics of the polyether.
Combs and McDaniel in U.S. Pat. No. 6,821,308 teach the value of low unsaturation monols for use in fuel additive applications. In the background section of the '308 patent, polyethers terminated with an alkyl group ranging from C9C30 (more preferably) are said to have better solubility and compatibility with fuels. The products from Stosser et al. (entries 2-7, in the above table) have monols in the range of 4 to 28.8 percent that are terminated with either C3 or C4 allyl or methallyl groups. Thus, in the worst case, 28.8 percent of the polyethers would be terminated with the C4 methallyl group instead of the more desirable C13 and the C4 group would decrease the compatibility with hydrocarbon fuels in comparison with a C13-terminated polyether.
The addition of acids to facilitate other aspects of DMC-catalyzed processes is known to those skilled in the art (See, U.S. Pat. No. 6,077,978 and EP 1 577 334). The addition of acid in these processes is reported to enhance process stability and to allow certain low molecular weight starters to be used either in the continuous addition of starter (“CAOS”) processes or in processes in which the starter is continuously added to the reactor during some part of the alkoxylation process. No effect on polyether unsaturation is noted in these references. Browne, in U.S. Published Patent Application 2005-0209438 A1, discloses the addition of acid to low molecular weight starter feed streams in a DMC-catalyzed CAOS process.
Although high unsaturation products are currently acceptable in fuel additive applications, the stringent emission and performance requirements of today's advanced engines can more easily be met with high performance polyethers that contain lower amounts of the unsaturated by-products. In addition, emission and performance requirements of tomorrow's engines will likely be more stringent and thus more difficult to satisfy with the currently available high unsaturation products. It would be desirable to have polyol production processes that can be used to prepare such polyether polyols with low unsaturation values from any source of alcohol.